1. Field of the Invention
The present invention relates to an optical head apparatus and an optical information apparatus for recording/reproducing or erasing information recorded on an optical information medium such as an optical disk, a recording/reproducing method in an optical information apparatus, and a system adopting them. Furthermore, the present invention relates to an objective lens (optical lens) and a diffraction element used in the optical head apparatus, and a phase difference.
2. Description of the Related Art
An optical memory technique using an optical disk having a pit pattern as a recording medium with high density and high capacity is being widely used for a digital audio disk, a video disk, a document file disk, a data file, and the like, and is being put into practical use. The function of conveniently recording/reproducing information with respect to an optical disk with high reliability through a minutely condensed light beam is classified into a condensing function of forming diffraction-limited minute spots, focus control (focus servo) and tracking control of an optical system, and a pit signal information signal) detection.
Recently, due to the advancement of an optical system design technique and a decreased wavelength of a semiconductor laser used as a light source, an optical disk having a high-density storage capacity larger than that of a conventional example has been developed. As an approach to higher density, an increase in a numerical aperture (NA) on an optical disk side of a condensing optical system that condenses a light beam minutely onto an optical disk has been studied.
In this case, there is a problem that the generated amount of aberration is increased due to the incline (so-called tilt) of an optical axis. When the NA is increased, the amount of aberration occurring with respect to tilt increases. In order to prevent this, the thickness of a substrate (substrate thickness) of an optical disk may be decreased.
A compact disk (CD) that may be called a first-generation optical disk uses infrared light (wavelength λ3: 780 nm to 820 nm) and an objective lens with an NA of 0.45, and has a substrate thickness of 1.2 mm. A DVD that is a second-generation optical disk uses infrared light (wavelength λ2: 630 nm to 680 nm; standard wavelength: 650 nm) and an objective lens with an NA of 0.6, and has a substrate thickness of 0.6 mm. A third-generation optical disk uses blue light (wavelength λ1: 390 nm to 415 nm; standard wavelength: 405 nm) and an objective lens with an NA of 0.85, and has a substrate thickness of 0.1 mm.
In the specification, the substrate thickness refers to a thickness from a surface of an optical disk (or an information medium) upon which a light beam is incident to an information recording surface.
Thus, the substrate of a high-density optical disk is designed to be thin. In view of cost efficiency and an occupied space of an apparatus, there is a demand for an optical information apparatus capable of recording/reproducing information with respect to optical disks that are varied in substrate thickness and recording density. For this purpose, an optical head apparatus is required to be provided with a condensing optical system capable of condensing a light beam up to a diffraction-limited beam onto optical disks having different thicknesses.
Furthermore, in the case of recording/reproducing information with respect to a disk with a thick substrate, it is necessary to condense a light beam onto a recording surface placed on an inner side of a disk surface. Therefore, a focal length must be set to be larger.
In view of the above, a configuration is disclosed, which is intended for compatibly reproducing information from different kinds of optical disks with light beams having a plurality of wavelengths. This will be described with reference to FIG. 20.
In FIG. 20, reference numerals 10 and 11 respectively denote optical disks having a transparent substrate thickness of 0.1 mm (t1) and 0.6 mm (t2). In order to enhance stiffness, a protective substrate is attached to a reverse surface (on an opposite side of an objective lens 40) of a transparent substrate, which is omitted in FIG. 20.
The objective lens 40 includes a refractive lens 402 on one surface 403 of which a layer 401 made of a different material from that of the refractive lens 402 is cemented. The objective lens 40 utilizes the difference in refractive index and dispersion between the refractive lens 402 and the layer 401. The objective lens 40 allows light beams having different wavelengths to be incident. The objective lens has spherical aberration characteristics in which a spherical aberration is changed to an underside when the wavelength of a luminous flux from a light source is shifted toward a long wavelength side. The spherical aberration displaced to the overside is cancelled by the spherical aberration displaced to the underside due to light having a longer wavelength. Thus, compatible recording/reproducing of optical disks having different thicknesses is made possible (see, for example, JP 2002-237078 A (pages 6–7, FIG. 1)).
As a second conventional example, a configuration in which a wavelength selecting phase plate is combined with an objective lens is disclosed. This will be described with references to FIGS. 21 and 22A–22B. FIG. 21 shows a schematic configuration of an optical head apparatus. Collimated light output from a blue light optical system 51 having a blue light source with a wavelength λ1 of 405 nm passes through a beam splitter 161 and a wavelength selecting phase plate 205, and is condensed onto an information recording surface of an optical disk 9 (third-generation optical disk) with a substrate thickness of 0.1 mm by an objective lens 50.
The light reflected from the optical disk 9 follows an opposite path to be detected by a detector of the blue light optical system 51. Diffused light output from a red light optical system 52 having a red light source with a wavelength λ2 of 650 nm is reflected from the beam splitter 161 and passes through the wavelength selecting phase plate 205, and is condensed onto the information recording surface of an optical disk 10 (second-generation optical disk: DVD) with a substrate thickness of 0.6 mm by the objective lens 50. The light reflected from the optical disk 10 follows an opposite path to be detected by a detector of the red light optical system 152.
The objective lens 50 is designed so as to allow collimated light to pass through the optical disk 9 with a substrate thickness of 0.1 mm to be condensed, when the collimated light is incident, and a spherical aberration occurs due to the difference in substrate thickness during recording/reproducing of a DVD. In order to correct such a spherical aberration, a light beam output from the red light optical system 52 and incident upon the objective lens 50 is formed into diffused light, and the wavelength selecting phase plate 205 is used. When diffused light is incident upon the objective lens 50, a new spherical aberration occurs.
Therefore, the spherical aberration occurring due to the difference in substrate thickness is cancelled with the new spherical aberration, and the wavefront is corrected by the wavelength selecting phase plate 205.
FIG. 22A is a plan view showing the wavelength selecting phase plate 205, and FIG. 22B is a sectional side view thereof. The wavelength selecting phase plate 205 satisfies a relationship: h=λ1/(n1−1) (where λ1 is a wavelength; n1 is a refractive index, and h is a height), and has a phase level difference 205a of h and 3h. The difference in optical path length caused by the height h with respect to light having the wavelength λ1 is a wavelength λ1, corresponding to a phase difference 2π radian, which is the same as a phase difference 0.
Therefore, a phase distribution is not influenced, and recording/reproducing of the optical disk 9 is not influenced. On the other hand, regarding light having the wavelength λ2, assuming that the refractive index of the wavelength selecting phase plate 205 is n2 at the wavelength λ2, h/λ2×(n2−1) is about 0.6 λ (i.e., the difference in optical path length that is not an integral multiple of a wavelength occurs). The above-mentioned aberration correction is performed by using a phase difference due to the difference in optical path length (see, for example, JP 10(1998)-334504 A (pages 7–9, FIGS. 1–4) and ISOM2001TECHINICAL DIGEST Session We-C-05 (page 30 of Preprints)).
Furthermore, as a third conventional example, a configuration is disclosed in which a plurality of objective lenses are switched mechanically. (See, for example, JP11(1999)-296890 A (pages 4–6, FIG. 1)).
Furthermore, as a fourth conventional example, a configuration is disclosed in which a mirror with a reflective surface having different radii of curvature also functions as a mirror for allowing an optical axis to be deflected (See, for example, JP 11(1999)-339307 A (pages 4–5, FIG. 1)).
As a fifth conventional example, a configuration is disclosed in which a refractive objective lens is combined with a hologram in the same way as in the first conventional example, and the difference in substrate thickness is corrected using a chromatic aberration occurring due to diffracted light beams having different wavelengths and the same order (See, for example, JP 2000-81566 A (pages 4–6, FIGS. 1 and 2)).
The first conventional example has spherical aberration characteristics in which a spherical aberration is changed to an underside when the wavelength of a luminous flux from a light source is shifted toward a long wavelength side. The spherical aberration displaced to the overside is cancelled by the spherical aberration displaced to the underside due to light having a longer wavelength.
For example, when reproduction of information is switched to recording of information with respect to an optical disk having a transparent substrate with a thickness t1, the light amount needs to be increased by 10 times. A wavelength is changed to be long accordingly. Since the wavelength becomes longer, the spherical aberration is changed to an underside. However, the thickness of the disk is not changed. Therefore, an unintended spherical aberration occurs and the light condensing ability is degraded.
Furthermore, the change in wavelength due to the change in light amount also changes a focal length. In FIG. 3 of the first conventional example (JP 2002-237078 A), when the wavelength of blue light is changed by 10 nm, the focal length is changed by about 10 μm. In FIG. 4 of the first conventional example, when the wavelength of red light is changed by 10 nm, the focal length is changed by about 3 μm. Particularly, when the focal length of blue light is changed greatly, light condensing characteristics are degraded while an objective lens is moved by focus control immediately after the light amount is changed.
In the second conventional example, a wavelength selecting phase plate is used as a compatible element. When information is recorded/reproduced with respect to a disk having a large substrate thickness, a recording surface is positioned far away from an objective lens by a substrate thickness. Therefore, it is necessary to extend a focal length. The focal length can be extended by allowing the compatible element to have a lens power. The wavelength selecting phase plate has no lens power.
Furthermore, in the case where it is attempted to realize the above-mentioned lens power by forming red light into diffused light as in the second conventional example, a large aberration occurs when the objective lens is moved by tracking or the like, and consequently, recording/reproducing characteristics are degraded.
Furthermore, the collimation degree of light that is reflected from an optical disk and returns through an objective lens is varied depending upon the disk substrate thickness. Therefore, a detection lens and a photodetector cannot be configured as one unit, and they must be prepared separately in accordance with the collimation degree of light.
In the third conventional example, objective lenses are switched. Therefore, a plurality of objective lenses are required, and the number of components is increased. Furthermore, it is difficult to miniaturize an optical head apparatus. It also is difficult to miniaturize an apparatus since a switching mechanism is required.
In the fourth conventional example, an objective lens is driven independently from a mirror (See FIGS. 4–6 of JP 11(1999)-339307 A). However, a light beam is converted from collimated light by a mirror having a radius of curvature as described above. Therefore, when the objective lens is moved by track control, the relative position of the objective lens with respect to an incident light wavefront is changed to cause an aberration, which degrades light condensing characteristics.
Furthermore, the reflective surface of a mirror is composed of a surface with a radius of curvature, i.e., a spherical surface. However, the spherical surface is insufficient for correcting the difference in substrate thickness and the difference in wavelength, and high-order (5th-order or more) aberration cannot be reduced sufficiently.